Conventional apparatuses, which estimate the rotational position of a predetermined magnetic pole of a motor having salient pole characteristic as an example of rotary electric machines, have been widely used. One example of these apparatuses, which uses the salient pole characteristic of the rotor, is disclosed in Japanese Patent Publication No. 3312472, referred to as a published patent document.
Note that the rotational position of a predetermined magnetic pole of a motor represents the actual rotational position of the direction of a magnetic field, i.e. a magnetic flux, generated by a predetermined magnetic pole, i.e. an N pole, of the rotor of the motor.
Specifically the apparatus disclosed in the published patent document applies an alternating-current (AC) voltage vector to the motor. This application generates a first current component in a first direction perpendicular to the AC voltage vector and a second current component in a second direction parallel to the AC voltage vector except when the AC voltage vector is perpendicular or parallel to γ axis. The γ axis represents an estimated position of a predetermined magnetic pole of the rotor. The apparatus also measures the first and second current components, and estimates, according to at least one of the measured first and second current components, the actual position of the predetermined magnetic pole of the rotor using the salient pole characteristic of the rotor. Note that the salient pole characteristic of the rotor represents the difference in reluctance between the actual direction of the magnetic field generated by the predetermined magnetic pole and the direction magnetically perpendicular to the actual direction of the magnetic field generated by the predetermined magnetic pole.